The invention generally relates to a method for ultrasound testing of a series of spot welding joints of a selected type and is directed to improvement within such type testing methods, namely to determining threshold values for gates which permit to evaluate the quality of a sound coupling. The invention is primarily concerned with determining temporal threshold values and also proposes amplitude threshold values.
Methods for ultrasound testing of spot welding joints are well known, the reader being referred in this context to U.S. Pat. Nos. 4,208,917; 4,265,119 and 5,537,875.
The known methods may be divided into methods in which ultrasound testing is performed substantially concurrently with the making of the spot welding joints and into such methods by which an already made spot welding joint is tested subsequently by means of ultrasound. The invention only relates to the second type of method, meaning to subsequent testing.
The ultrasound testing of spot welding joints is based on the evaluation of the echo sequence of an ultrasound signal emitted into the spot welding joint, preferably of a pulse. Typically, a sequence of pulses is emitted. Heavily damped transmit/receive transducer probes or heads, which usually make use of film resonators and have a frequency typically ranging from 15 to 20 MHz, are utilized. The diameters of the transducers are approximately within the range of the diameter of the spot welding joints. Coupling typically occurs through a preliminary water stretch that is often sealed against the outside by a thin membrane. Direct contact of the water with the metal sheets is thus avoided.
Presently, ultrasound testing of spot welding joints is still mainly carried out by hand. Automation of testing is being strived for. The invention is intended to both simplify manual testing and to indicate ways for automatic testing.